CSS class migrations: admin views and boot overlay fade-out

- Replace style.display with .is-hidden classList in ApiKeysView, CustomFieldsView, RecurringTicketsView
- Convert boot overlay fade-out from style.opacity to .boot-overlay--fade-out CSS class
- Add .boot-overlay--fade-out rule to dashboard.css

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/admin/CustomFieldsView.php

@@ -123,7 +123,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
                             <option value="number">Number</option>
                         </select>
                     </div>
-                    <div class="setting-row" id="options_row" style="display: none;">
+                    <div class="setting-row is-hidden" id="options_row">
                         <label for="field_options">Options (one per line)</label>
                         <textarea id="field_options" name="field_options" rows="4" placeholder="Option 1&#10;Option 2&#10;Option 3"></textarea>
                     </div>
@@ -208,7 +208,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
 
         function toggleOptionsField() {
             const type = document.getElementById('field_type').value;
-            document.getElementById('options_row').style.display = type === 'select' ? 'block' : 'none';
+            document.getElementById('options_row').classList.toggle('is-hidden', type !== 'select');
         }
 
         function saveField(e) {